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Problem with init debugging under QEMU ARM
From: |
Lukasz Majewski |
Subject: |
Problem with init debugging under QEMU ARM |
Date: |
Mon, 13 Sep 2021 10:29:17 +0200 |
Dear QEMU community,
I'm now trying to fix bug in glibc which became apparent on qemu 6.0.0.
The error is caused by glibc commit:
bca0f5cbc9257c13322b99e55235c4f21ba0bd82
https://sourceware.org/git/?p=glibc.git;a=blobdiff;f=sysdeps/arm/dl-machine.h;h=eb13cb8b57496a0ec175c54a495f7e78db978fb7;hp=ff5e09e207f7986b1506b8895ae6c2aff032a380;hb=bca0f5cbc9257c13322b99e55235c4f21ba0bd82;hpb=34b4624b04fc8f038b2c329ca7560197320615b4
(reverting it causes the board to boot again)
Other components:
binutils_2.37
gcc_11.2
Yocto poky: SHA1: 1e2e9a84d6dd81d7f6dd69c0d119d0149d10ade1
Qemu start line (the problem is visible on 5.2.0 and 6.0.0):
qemu-system-arm -device
virtio-net-device,netdev=net0,mac=52:54:00:12:34:02 -netdev
tap,id=net0,ifnam e=tap0,script=no,downscript=no -object
rng-random,filename=/dev/urandom,id=rng0 -device
virtio-rng-pci,rng=rng0 -drive
id=disk0,file=y2038-image-devel-qemuarm.ext4,if=none,format
=raw -device virtio-blk-device,drive=disk0 -device qemu-xhci -device
usb-tablet -device usb-kbd -machine virt,highmem=off -cpu cortex-a15
-smp 4 -m 256 -serial mon:stdio -serial null -nographic -device
VGA,edid=on -kernel
zImage--5.10.62+git0+bce2813b16_machine-r0-qemuarm-20210910095636.bin
-append 'root=/dev/vda rw mem=256M
ip=192.168.7.2::192.168.7.1:255.255.255.0 console=ttyAMA0 console=hvc0
vmalloc=256
It has been tested with Cortex-A9 (Vexpress A9 2 core board) and
Cortex-A15. I've also tested the v5.1, v5.10 and v5.14 kernels. The
error is persistent.
I do add -s and -S when starting qemu-system-arm. I can use gdb to
debug the kernel without issues. Unfortunately, I'm not able to debug
/sbin/init after switching contex to user space.
Moreover, gdbserver cannot be used as the error (and kernel OOPs) is
caused when early code from ld-linux.so.3 (the _dl_start function) is
executed.
Any hints on how to debug it?
Inspecting assembler is one (awkward) option (some results presented
below). I can also inspect the VMA of the code just before starting the
/sbin/init process.
Unfortunately, when I try to break on user space code it is not very
helpful (as -S -s are supposed to be used with kernel).
Some info with the eligible code (_dl_start function):
------------------------------------------------------
I think that the problem may be with having the negative value
calculated.
The relevant snipet:
116c: bf00 nop
116e: bf00 nop
1170: bf00 nop
1172: f8df 3508 ldr.w r3, [pc, #1288] ; 167c
<_dl_start+0x520>
1176: f8df 1508 ldr.w r1, [pc, #1288] ; 1680
<_dl_start+0x524>
117a: 447b add r3, pc
117c: 4479 add r1, pc
117e: f8c3 1598 str.w r1, [r3, #1432] ; 0x598
1182: bf00 nop
1184: bf00 nop
1186: bf00 nop
1188: bf00 nop
118a: bf00 nop
118c: bf00 nop
118e: f8df 24f4 ldr.w r2, [pc, #1268] ; 1684
<_dl_start+0x528> 1192: f8d3 5598 ldr.w r5, [r3,
#1432] ; 0x598 1196: 447a add r2, pc
1198: 442a add r2, r5
119a: 1a52 subs r2, r2, r1
119c: f8c3 25a0 str.w r2, [r3, #1440] ; 0x5a0
11a0: 6813 ldr r3, [r2, #0]
167c: 0002be92 .word 0x0002be92
1680: ffffee80 .word 0xffffee80
The r1 gets the 0xffffee80 (negative offset) value. It is then added to
pc and used to calculate r2.
For working code (aforementioned patch reverted) - there are NO such
large values (like aforementioned 0xffffee80). The arithmetic is done
on
1690: 00000020 .word 0x00000020
1694: 0002be7e .word 0x0002be7e
which seems to work.
Maybe I'm missing some flag when I do start qemu-system-arm?
Thanks in advance for help and hints.
--
Best regards,
Łukasz Majewski
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- Problem with init debugging under QEMU ARM,
Lukasz Majewski <=